Vinyl acetate monomer (VAM) is prepared in a continuous process with recycling of the purified product stream (cycle gas system). In a heterogeneously catalysed gas phase process, ethylene reacts with acetic acid and oxygen over fixed bed catalysts which generally comprise palladium and alkali metal salts on a support material and may additionally be doped with gold, rhodium or cadmium.
The ethylene, oxygen and acetic acid reactants are reacted in an exothermic reaction (VAM: ΔBH°299=−176 kJ/mol), generally at a pressure of 1 to 30 bar and a temperature of 130° C. to 200° C., in a fixed bed tubular reactor to give vinyl acetate monomer:C2H4+CH3COOH+½O2→CH3COOCH═CH2+H2O
The main side reaction is the total oxidation of ethylene to CO2:C2H4+3O2→2CO2+2H2O
The heat of formation here is ABH°299=−1324 kJ/mol!
The ethylene conversion is about 10%, the acetic acid conversion 20 to 30%, and the oxygen conversion up to 90%.
In the preparation of vinyl acetate monomer, a gas mixture consisting predominantly of ethylene, carbon dioxide, ethane, nitrogen and oxygen is circulated. Upstream of the fixed bed tubular reactor, the gas stream is admixed with the acetic acid, ethylene and oxygen reactants, and brought to reaction temperature with steam-operated heat exchangers. The cycle gas is enriched with acetic acid typically by means of a steam-heated acetic acid saturator or acetic acid evaporator.
After the reaction, the reaction products and unconverted acetic acid are condensed out of the cycle gas and sent to workup. Uncondensed product is scrubbed out in a scrubber operated with acetic acid. The cycle gas or a portion thereof, before being admixed again with the reactants, is purified to free it of carbon dioxide formed. The condensed vinyl acetate monomer and water products, and also unconverted acetic acid, are separated from one another in a multistage distillation process typically operated with steam. The customary distillation steps are dewatering (optionally also preliminary dewatering), azeotropic distillation, pure VAM column, residue workup, and low boiler and high boiler removal.
The reaction temperature in the fixed bed tube bundle reactor, generally from 130° C. to 200° C., is set by means of evaporative water cooling at a pressure of 1 to 10 bar. This forms steam, known as in-process steam, generally at a temperature of 120° C. to 185° C., at a pressure of 1 to 10 bar, preferably 2.5 to 5 bar. The decline in activity over the operating time of a catalyst is balanced by increasing the reaction temperature, i.e. the operating pressure of the evaporative water cooling. To conserve the catalyst, for the purpose of achieving a maximum service life, and to optimize the ethylene selectivity, by minimizing the CO2 formation, the vinyl acetate reaction should be conducted for as long as possible with as low as possible a reaction temperature.
In general, the fixed bed tube bundle reactors are formed from several thousand, typically 2000 to 6000, tightly packed and vertically arranged, cylindrical tubes. For industrial scale use, tubes with a length of 5 to 6 m and an internal diameter of 33 mm to 40 mm are used for this purpose. The surface/volume ratio is generally 100 to 120 m−1.
A problem in vinyl acetate monomer preparation in a heterogeneously catalysed gas phase process, especially given increasingly high-performance catalysts, is the heat of reaction released. This is because the process is characterized by significant exothermicity, to a lesser degree in the main reaction than in the side reaction of ethylene oxidation. Even given good ethylene selectivity values (>92%), at a molar ratio of VAM: CO2 of approx. 7:1, owing to the high heat of formation of the CO2, half of the heat of reaction originates from this side reaction! Even given the constant improvement in VAM catalysts with regard to the space-time yield (STY), it is therefore never possible at the same time also to improve the ethylene selectivity SE in such a way that, as a result of the increase in STY, the overall exothermicity would also be reduced.
The procedure in the prior art to date has been to compress to a higher degree with higher-performance cycle gas compressors, in order that the flow rate of the cycle gas was increased, and the axial heat removal was optimized. Disadvantages are the higher pressure differences in all cycle gas apparatuses, and the difficulty of controlling the high gas velocity of the cycle gas. For the conventional VAM catalysts, the STY of which is between 400 and 700 g of VAM/l of catalyst×h, such measures, with all the disadvantages mentioned, were sufficient.
In the case of high-performance catalysts for VAM preparation, the STY of which is significantly above 700 g of VAM/l of catalyst×h, the heat of reaction formed is significantly higher, with the consequence that the removal of heat in the steam/water cooling medium, even with the abovementioned measures for increasing the cycle gas velocity, is too slow. The consequences are that there is not just increased occurrence of local temperature increases (hotspots) in the reactor and an associated reduction in ethylene selectivity, but also shortened catalyst service lives and higher by-product formation overall.
EP 1033167 A2 discloses a tube bundle reactor with graduated internal diameter, wherein the tubes in the region of the inlet of the reaction mixture have a smaller diameter than in the region of the outlet of the reaction mixture. The document advises against a general reduction in the diameter of the tubes of a tube bundle reactor, since the manufacturing costs of the reactor (more tubes) would thus increase and the expenditure associated with filling them would rise.